![]() ![]() ![]() If this is just for experimenting, use a 100-500 kΩ pot for the two bias resistors and adjust it so that the drain sits at about half the supply voltage (4.5 V) when there is no input signal. It might be surprising, but FET technology was invented in 1930, some 20 years before the bipolar transistor. Because of that, the bias point produced by the two 100 kΩ resistors can't be computed up front. MOSFET is an acronym for Metal Oxide Semiconductor Field Effect Transistor and it is the key component in high frequency, high efficiency switching applications across the electronics industry. MOSFETs don't have as well a defined gate voltage threshold where they quickly change between off and on. This circuit could work with a N channel MOSFET replacing the darlington, but the same output power calculations still apply. This circuit is so poor that the additional distortion caused by DC thru the speaker is the least of the problems. That's generally not a good idea, but at these low power levels won't cause any harm to a normal size speaker. With a typical 8 Ω speaker, the output power will be considerably lower than that.Īnother problem with this circuit is that it puts DC current thru the speaker. ![]() Or about 1/20 of the advertised output power. That means the load (the speaker) will see half the open circuit RMS voltage, which is 1.6 V. The highest power transfer happens when the load impedance matches the source impedance, in other words when the speaker is also 50 Ω. My understanding is that the BJT is faster if you DON'T switch the transistor hard enough to saturate, whereas the MOSFET is faster if you DO switch the transistor hard enough to saturate. Even ignoring the inevitable off-center biasing of the transistor and any voltage drop accross it, at best this circuit is a 4.5 Vpp AC source with 50 Ω output impedance. 1 Yep it's conflicted because few talk about the actual type of switching that is going on. It is in no way capable of providing 1 W of AC power into the speaker. This is a pretty crappy circuit in the first place. This non-linear relationship between RDS(on) and V(BR)DSS is a compelling reason to research ways to reduce the conduction loss of power transistors 2. BJTs simple way to think of the BJT is as two diodes back to back in either a PN to NP or NP to PN configuration, thereby forming a PNP junction or NPN junction. MOS V and Power MOS 7 MOSFETs reveals that RDS(on) increases as the square of V(BR)DSS. This is an important distinction to keep in mind throughout this paper. Bipolar transistors are cheap and available. A BJT is driven by current, whereas the MOSFET and IGBT are voltage driven. ![]()
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